1. Field of the Invention
This invention relates to a scanning line converting system for displaying a high definition television system video signal on a television receiver by reducing the number of scanning lines. More particularly, it is directed to a scanning line converting system for displaying, via non-interlaced scanning, the high definition television signal transmitted via interlaced scanning.
2. Description of the Related Art
A high definition television system, such as a "high vision" system, has been proposed as a new television system for replacing the current television system. Such a high definition television system is capable of not only improving the quality of the picture and sound, but is also capable of providing powerful visuo-psychic effects with far more realistic and sharper pictures than the current system. A MUSE (Multiple Sub-Nyquite Sampling Encoding) system is a transmission system proposed for the high definition television signals.
Since the MUSE system differs greatly in the screen, scanning, and audio processing from the current television system, such as an NTSC (National Television System Committee) system, it is not possible to reproduce the MUSE signal directly on NTSC-based television receivers. For this reason, a MUSE/NTSC converter (down-converter) has been proposed to convert a MUSE signal to an NTSC signal for display on the receiver by cancelling out the differences, such as the number of scanning lines, aspect ratio, field frequencies, and the like, existing between the two systems.
FIG. 6 shows diagrams of examples of images in the output of a down-converter. FIG. 6(a) shows an output mode, whereby 1032 effective scanning lines of a MUSE signal are reduced by about a third so as to be allocated to 344 NTSC-based effective scanning lines without changing the aspect ratio of 16:9 of the MUSE system (hereinafter referred to as "full-screen mode"). In this mode, about 30% of the effective scanning lines are blanked out on both the top and bottom of the screen, with the horizontal screen being the same as in the original MUSE-based screen.
FIG. 6(b) shows another output mode, whereby 1032 effective scanning lines of the MUSE signal are reduced by about a half so as to be allocated to 483 NTSC-based effective scanning lines without changing the aspect ratio of 4:3 of the NTSC system, thereby deleting about 30% of picture elements from both left and right sides of the image (hereinafter referred to as "left-and-right-side cut mode"). In this mode, a portion to be deleted can be determined arbitrarily.
By way of background information, in the current television system represented by the NTSC system, a number of methods have been developed to reduce the bandwidth. One of such methods is interlaced scanning. This scanning is a system in which one complete screen (one frame) is formed by two roughly scanned screens utilizing the afterimage effect of the eye or television receiver. The odd-numbered scanning lines are transmitted in the first field and the even-numbered scanning lines in the second field to form and thereby display one frame of a picture. In comparison with the non-interlaced scanning in which all the scanning lines of a video signal are transmitted, that is, where the video signal is displayed with one scanning, interlaced scanning provides the advantage in that the frequency bandwidth of a video signal can be reduced by half without impairing the picture quality.
However, interlaced scanning has the problems of interline flicker and reduced vertical resolution because a delay of 1/30 seconds occurs between two odd-numbered fields which causes the image of the even-numbered field displayed in between to interfere with the picture. Especially in recent years, the introduction of high-luminance cathode-ray tubes (CRT) has aggravated the problem of interline flicker which has been negligible in low-luminance CRTs. The flicker effect becomes more objectionable with increased luminance and screen size.
Thus, to improve the picture quality, it is conceivable to improve the vertical resolution by converting an interlaced scanning signal to a non-interlaced scanning signal. Particularly, it is preferable to employ non-interlaced scanning when displaying the high-vision MUSE signal converted to the NTSC signal. However, interpolation of scanning lines of the once-NTSC converted signal for display via non-interlaced scanning appreciably impairs the high-quality picture. Further, the receiver, which is to display the NTSC-converted signal, functions merely in the ordinary NTSC-based non-interlaced display mode, and is thereby lacking in such functions as adaptation processing and interpolation processing required for MUSE/NTSC conversion and display. This results in complicating the design and increasing the cost.